Xerographic charging apparatus with means to automatically control the potential applied to the corona wire



Aug. 8, 1967 J COD|CH|N| ET AL 3,335,274

XEROGRAPHIC CHARGING APPARATUS WITH MEANS TO AUTOMATICALLY CONTROL THE POTENTIAL APPLIED To THE CORONA WIRE Filed Dec. 4, 1964 2 Sheets-Sheet l //v VE/V 701% ATTORNEYS Aug. 8, 1967 J CQDICHIN] ET AL 3,335,274

XEROGRAPHIC CHARGING APPARATUS WITH MEANS TO AUTOMATICALLY CONTROL THE POTENTIAL APPLIED TO THE CORONA WIRE 2 Sheets-Sheet 2 Filed Dec. 4, 1964 S RmM mwE G MIR 4 WM m Pw J EE N O w. G U y E v/ Mm B UT m E N E G FIG. 4

DC. POWER SOURCE ATTORNEYS United States Patent 3,335,274 XEROGRAPHIC CHARGING APPARATUS WITH MEANS T0 AUTOMATICALLY CONTROL THE POTENTIAL APPLIED TO THE CORONA WIRE Joseph J. Codichiui, Kennett Square, Pa., and Eugene W.

Yurgealitis, Webster, N.Y., assignors to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Dec. 4, 1964, Ser. No. 415,967 2 Claims. (Cl. 250-495) ABSTRACT OF THE DISCLOSURE A circuit for controlling charging of an electrostatic plate having a resistor connected in series with a voltage reference device and the plate and a circuit for controlling the charging current in accordance with variations of the voltage drop developed across'the resistor.

This invention relates to the field of xerography and, particularly, to an improved electric circut to control a corona generating device for applying electrostatic charge on a xerographic plate.

By present techniques, the charging of the xerographic plate in preparation for the exposure step is accomplished by means of a corona generating device whereby an electrostatic charge on the order of 500 to 600 volts is applied to the xerographic plate. A form of corona generating device for this purpose is disclosed in Walkup Patent 2,777,957, issued Jan. 15, 1957, wherein a plurality of parallel wires are connected in series to a high voltage source and are supported in a conductive shield that is arranged in closely spaced relation to the surface to be charged. When the wires are energized, corona is generated along the surface of the wires and ions are caused to be deposited on the adjacent photoconductive surface. Suitable means are usually provided to effect relative movement of the surface to be charged and the corona generating device. A biased wire shield placed between the corona wires and the xerographic plate permits energizing the corona wires to a potential well above the corona threshold potential thereof without causing damage to the xerographic plate because the excess of corona current over that required for proper charging of the plate is drained off by the biased shield.

As is well known, the corona threshold potential and the corona current from an energized wire are functions of the wire diameter, i.e., the corona threshold increases and the corona current for any given potential decreases as the wire diameter is increased. Variations in the potential applied to corona wires of a given diameter will cause relatively large changes in corona current with corresponding variations in the charging rate. In addition, the corona threshold potential and corona current are also affected directly by deposits of dust that may accumulate on the wire, by atmospheric conditions such as humidity, temperature and pressure, and by variations of movement and ionized conditions of the air sheath surrounding the wire. Thus when operating at the corona threshold, minute differences in wire diameter, slight accumulations of dust on the wire, and variations in air current, atmospheric conditions and the spacing between the wire and the xerographic plate drastically affect the corona generating potential of the wire and cause a nonuniform electrostatic charge to be deposited on the xerographic plate.

In the art of xerography it has been established that consistently high quality reproductions can best be effected when a uniform potential is applied to a xerographic plate to prepare the plate for the exposure step. If the xerographic plate is not charged to a sufficient potential, the

electrostatic latent image obtained upon exposure will be relatively weak and the resulting deposition of a developer material thereon will be correspondingly small and, if the xerographic plate is overcharged, the converse will occur, and if overcharged sufficiently, the photoconductive layer of the xerographic plate may be permanently damaged.

Since the contrast value, comparable to the contrast values obtainable from silver halide papers, of the electrostatic latent image is related directly to the potential charge on the xerographic plate before exposure, it is apparent that if the plate is not uniformly charged over its entire area, the contrast value of the electrostatic latent image obtained upon exposure will vary in different areas on the plate, and a streaky effect will be visible on the image when developed.

It is, therefore, the principal object of this invention to improve the electrical circuit of a corona generating device whereby a uniform electrostatic charge may be deposited on a xerographic plate.

A further object of this invention is to improve a scorotron control circuit for use in automatic xerographic machines wherein it is desirable to continuously charge a xerographic plate to a uniform potential regardless of variations in the supply line voltage, or changes in the surrounding atmospheric conditions, structural variations of the control circuit elements, unevenness in a xerographic plate or variations in the spacing between the xerographic plate and the charging devices.

These and other objects of the invention are attained by cooperative action with a scorotron comprising a backup plate, coronode wires extending parallel to the backup plate to charge a xerographic plate by corona discharge and a screen partially enclosing the Wires whereby the potential applied to the xerographic plate may be varied by changing the screen or shield potential. The novel charging circuit for ensuring a constant charging current includes a resistor connected in series with a reference voltage device and the xerographic plate and a circuit arrangement for controlling the charging current in accordance with variations of the voltage drop developed across the resistor.

For a better understanding of the invention as well as other objects and features thereof, reference is had to the following detailed description of the invention to be read in connection with the accompanying drawings, wherein:

FIG. 1 illustrates schematically a preferred embodiment of a xerographic reproducing apparatus adapted for continuous and automatic operation, and incorporating a corona generating apparatus in accordance with the invention;

FIG. 2 is a perspective view, partly broken away to show structural details, of a preferred scorotron charging apparatus;

FIG. 3 is an end view of the scorotron charging apparatus; and,

FIG. 4 is a schematic electrical wiring diagram of the control circuit for the scorotron of FIG. 2.

Referring now to the drawings, there is disclosed a prefered embodiment of a variable speed xerographic processor for producing continuous tone images using a corona generating device of the invention. Specifically, the apparatus shown is a close-access xerographic recorder for producing permanent continuous tone images on a transfer material from suitable light images of the objects to be reproduced.

A shown in FIG. 1, the variable speed xerographic apparatus comprises a xerographic plate including a photoconductive layer or radiation-receiving surface on a conductive backing and formed in the shape of a drum, generally designated 10, which is mounted on a shaft journaled in a suitable frame to rotate in the direction indicated by the arrow to cause the drum surface sequentially to pass a plurality of xerographic processing stations.

For driving the drum at a variable speed there is provided a suitably mounted motor 12 connected to a conventional variable speed drive 14, the output shaft of which is connected in a suitable manner (not shown) to drive the drum. The output speed of the variable speed drive is controlled by means of shaft 16 to vary the speed of the drum as desired.

For the purpose of the present disclosure, the several xerographic processing stations in the path of movement of the drum surface may be described functionally, as follows:

A charging station, at which a uniformelectrostatic charge is deposited on the photoconductive layer of the xerographic drum;

An exposure station, at which a light or radiation pattern of copy to be reproduced is projected onto the drum surface to dissipate the drum charge in the exposed areas thereof and thereby form a latent electrostatic image of the copy to be reproduced;

A developing station, at which a xerographic developing material including toner particles having an electrostatic charge opposite to that of the electrostatic latent image is cascaded over the drum surface, whereby the toner particles adhere to the electrostatic latent image to form a xerographic powder image in the configuration of the copy to be reproduced;

A transfer station, at which the xerographic powder image is electrostatically transferred from the drum surface to a transfer material or suport surface; and

A drum cleaning and discharge station, at which the drum surface is brushed to remove residual toner particles remaining thereon after image transfer, and at which the drum surface isexposed to a relatively bright light source to effect substantially complete discharge of any residual electrostatic charge remaining thereon.

The charging station is preferably located as indicated by reference character A. In general, the charging apparatus includes a corona discharge device 18 which consists of any array of one or more corona discharge electrodes that extend transversely across the drum surface and are energized from a high potential source and are substantially enclosed within a shielding member.

Next subsequent thereto in the path of motion of the xerographic drum is an exposure station B. This exposure station may be one of a number of types of mechanisms or members to expose the charged xerographic drum to a radiation image, which then causes a release of the charge on the drum in proportion to the radiation from the copy onto the surface of the drum. As shown, the exposure mechanism includes a cathode-ray tube 20 connected to a suitable electronic circuit, not shown or described since it forms no part of the instant invention. Images projected by the cathode ray tube onto the image mirror 22 are reflected onto object mirror 24 for projection onto the Xerographic drum, the entire projection system being enclosed in a suitable exposure housing 26 to exclude extraneous light.

Adjacent to the exposure station is a developing station C in which there is positioned a development mechanism, such as development electrode 28 which is mounted to maintain a uniform close spacing between its upper surface and the surface of the xerographic plate to form a development zone therebetween. The development electrode 28 is supplied with a powder cloud by powder cloud generator 30 through conduit 32. Neither the development electrode 28 nor the powder cloud generator 30 are described in detail herein since they form no part of the subject invention.

Positioned next and adjacent to the developing station is the image transfer station D which includes a web feeding mechanism to feed a web of paper or other suitable transfer material to the drum and a transfer mechanism to effect transfer of a developed xerographic powder image from the drum onto the transfer material. The sheet feeding mechanism includes a supply roll 34 for a web of transfer material 36 which is fed up. and over a transfer mechanism 38 into transfer contact with the drum and then down under a viewing platen 40 across a fusing apparatus, such as heat fuser 42 whereby the developed and transferred xerographic powder image on the transfer material is permanently fixed thereto from whence it is Wound onto take-up roll 44.

The. next and final station in the device is a drum cleaning station E, having positioned therein a plate cleaner 46 adapted to remove any powder remaining on the xerographic plate after transfer by means of brushes 48 driven by motor 50 and a light source 52 adapted to flood the xerographis plate with light to cause dissipation of any residual electrical charge remaining on the xerographic plate.

Suitable control means are used to actuate the drum development electrode, powder cloud generator, web feed mechanism, and the plate cleaning device.

Referring now to the subject matter of the invention, the electrostatic charging of the xerographic plate in preparation for the exposure step is accomplished by means of a corona generating device whereby an electrostatic charge is applied to the plate surface as it moves relative to the charging device. The potential applied to the plate is dependent upon the particular print contrast desired. Higher print contrasts require higher initial plate potentials.

To effect charging of the plate there is provided a corona generating device in the form of a scorotron and a charging circuit to supply electrical power to the corona generating device. The scorotron shown in FIG. 2 includes a back-up plate, wires called the coronode, and screen wires. The coronode wires, by corona discharge, charge the photoconductive surface of the xerographic plate. The potential applied to the plate surface can'be varied by changing the screen potential.

Specifically, the corona generating device 18 includes two terminal blocks 54 and 56 made of suitable insulating material and connected to opposite ends of a grid bar or back-up plate 58 by suitable means, such as screws 60. The terminal block 54 ism-ounted on one end of the grid bar 58 while the terminal block 56 extends through a suitable aperture (not shown) formed in the opposite end of the grid bar. Stretched between and attached by means of the studs 62 to the stepped portions of the terminal blocks are a plurality of fine, high voltage coronode wires 64 of continuous length. One end of the continuous length wire is secured by one of the screws 60 to the terminal block 54, the wire then extending between the terminal blocks parallel to the grid bar 58 to form three strands of wire, the opposite end of the wire ending at terminal block 56 where the wire extends through apertures 66 and 68 therein and is connected to a binding post 70 threaded into the bottom end of the terminal block 56.

To control or suppress the effective charging potential of wires 64 there is provided a pair of side plates 72 secured to opposite sidesof the terminal blocks, each of the side plates having converging portions 74 each arranged at an angle of approximately 45 to the side plate anda top portion partly extending over the top of the terminal blocks. The top portions of the side plates are spaced apart to afford a corona discharge opening 76 extending parallel and above the wires 64.

The corona discharge opening 76 is partly screened by grid wires 78 of continuous length electrically connected to the side plates 72, the grid wires extending between the terminal blocks to.which they are secured by means of the studs 62 in the top of these blocks. One end of the wire forming screen or grid wires 78 is connected by one of the screws 60 to the end of terminal block 56 and then connected to the side plates 72 from whence "the Wire is Wound on the studs of the terminal blocks to form, as shown, eight strands of wires of continuous length, the opposite end of the Wires extending through a second aperture 68 in terminal block 56 to be connected to a second binding post 80. The side walls and grid wires form a control shield around the coronode wires to suppress the effective charging potential of the coronode wires.

Both the wires 64 and 78 are made of any suitable noncorrosive material, such as stainless steel, having a uniform exterior. In the embodiment of the charging device shown, the diameter of wires 64 is approximately threeand-one-half thousandths of an inch and the diameter of wires 78 is approximately ten thousandths of an inch, it being apparent that other size wire can be used.

For supporting the charging device there is provided a U-shaped slide support 82 having attached on its bottom side, as seen in FIG. 3, at opposite ends thereof a pair of flanged mounting plates 84 which extend laterally across the support whereby the charging device can be suitably attached to a frame element of the xerographic apparatus. The support 82 has attached to its upper surface a pair of grid slide bars 86 which extend the length of the slide grid and are positioned at opposite ends thereof by means of screws and nuts 90. The undercut portions of the grid slide bars 86 are positioned to face inward and parallel to each other to form a modified T-slot to slidably receive the slide brackets 88 secured along with insulating bars 85 to the depending side portions of grid bars 86 by means of screws and nuts. To prevent longitudinal movement of the charging device, a support plate 92 adapted for connection in a suitable manner to a frame element of the machine is secured to the outer face of terminal block 56.

A clearer understanding of the operation of the charging apparatus and of its electrical controlling circuit can best be obtained by reference to the schematic wiring diagram of FIG. 4. The circuit in FIG. 4 is provided with a DO. power source PS-l which is connected to a source of alternating current, such as a commercial outlet of 110 volts, for producing a DC. potential in the range of approximately 6,000 to 11,000 volts. The positive terminal for the DC. source is connected by a conductor 100 to the plate 102 of a high voltage triode T-l which has its cathode 104 connected to the coronode wires 64. The negative terminal for the source is connected by a conductor 106 to the back-up plate 58 and to ground. The screen 78 is connected to a voltage regulator tube T2, which is preferably selected to maintain the potential of 600 volts on the screen 78.

The xerographic plate is connected to one end of a resistor 108 having its other end connected to a reference voltage device 110. The plate 10 is also connected to the grid 114 of an amplifier tube T3 by the conductor '112. The plate for the tube T3 is connected through a resistor 116 to the conductor 100 and also by a conductor 118 to the grid 120 of the tube T-l. The cathode 122 for the tube T3 is connected to the ground conductor 106 by way of a Zener diode D-1 and also to the conductor 100 through a resistor 124. Each of the cathodes 104, 122 is provided with suitable cathode heater circuits (not shown).

The reference voltage device 110 is preferably a commercially available tachometer generator which is designed to produce a given value of potential for a particular speed of rotation of the xerographic drum 10 and to vary this potential linearly with drum speed variations. The resistor 108 serves as a sensing element and the voltage across it is directly proportional to the charging current on the wires 64. Assuming that the drum is rotating at a constant speed to produce a fixed potential on the drum, the voltage drop across the resistor 108 will remain constant as long as all conditions such as temperature, humidity, pressure, voltage potential on the plate, spacing between the wires 64 and the plate 10, and the evenness of the plate and the wires 64 remain constant. In the event there is a change in any of these conditions, there will be a corresponding variation in the charging current on the wires 64 which, in turn, will cause a variation in the voltage drop across the resistor 108. The voltage drop across the combination of the resistor and the reference voltage device is continuously compared to the voltage produced by the Zener diode D-l. Any increase or decrease in the voltage drop will produce a differential voltage which will vary the voltage between the grid of tube T3 and the cathode therefor.

The resultant signal which is proportional to the differential voltage will be amplified by tube T-3 and applied to the grid of the tube T-1. This in turn develops a voltage drop across the tube T-1 to control the potential applied to the wires 64 and thereby correct the effect of that condition which originally produced the change in the voltage drop across the resistor 108 and the reference voltage device 110. In effect, if one of the above conditions causes a decrease in the charging current on the wires 64, there will be a proportional decrease in the voltage drop across the combination of the resistor 108 and the reference device 110. This voltage drop and the voltage drop across the Zener diode D-1 will be applied to the tube T-3 where the differential is amplified. The amplified differential will be applied to the grid of the tube T-l for increasing the conduction therethrough in order to increase the voltage applied to the wires 64.

This increase will remain until the voltage drop across the combination of the resistor 108 and the reference device 110 is increased to its original value.

In the event that the drum speed varies, which condition would produce an uneven corona discharge thereon, the reference voltage device 110 will produce a voltage corresponding to the change in drum speed and this would result in a differential voltage appearing on the grid of tube T-l having a value corresponding to the change in the potential upon the wires 64. With a new signal appearing on the grid of tube T-l, the conduction thereof will vary according to the differential voltage which in turn will vary the potential on the wires 64 to offset the effect produced by the drum speed variation.

The present invention may also be applied to a xerographic apparatus having a movable fiat xerographic plate instead of a rotating drum. In this use, the reference voltage device 110 will be operable upon the mechanism which moves the plate across the field produced by the wires 64. The speed of the moving plate should be made constant with the output voltage for the device 110 being preset in accordance with this speed. In all respects then, the present invention is applicable for xerographic apparatus having either a drum or moving flat plate.

With the control circuit of the invention, the charging current can be maintained at a constant value and is not influenced by any of the normal variables such as geometry of the scorotron, voltage variations, dirty wires, atmospheric changes, plate moving speed, evenness of the plate, etc., which ordinarily affect charging current. By maintaining the charging cun'ent constant, high quality reproduction having the desired contrast value can be made continuously and automatically.

While the invention has been described with reference to the circuit disclosed herein, it is not confined to the details set forth since it is apparent that certain electrical equivalent components may be substituted for the components of the preferred circuit without departing from the scope of the invention. This application is therefore intended to cover such modifications or changes as may come within the purposes of the inventon as defined by the following claims.

What is claimed is:

1. In a xerographic reproducing apparatus having a corona generating device with at least one corona wire positioned in closely spaced relation to a xerographic plate for applying an electrostatic charge onto the xerographic plate, and wherein the charging current to the xerographic plate is produced by the potential applied to the corona wire whereby an increase in potential on this wire will effect an increase in charging current and a decrease in this potential will effect a decrease in charging current;

a circuit for the corona generating device including a high voltage direct current source,

a control tube having a cathode, an anode and 'a grid, said cathode being electrically connected to said wire, said anode being electrically connected to the positive terminal of said source,

an electrical connection between the xerographic and the negative terminal of said source,

a resistor electrically connected in series between the Xerogr-aphic plate and said negative terminal whereby the charging current through said xerographic plate flows through said resistor, said resistor being adapted to produce a voltage directly proportional to the charging current, and

circuit means electrically coupled between said resistor and said grid whereby the variations in the charging,

current, as determined by the voltage across ,said resistor, causes variations in the voltage applied between the grid and cathode of said tube thereby changing the, voltage applied to said wire to correct for the variations in the charging current.

2. In a xerographic reproducing apparatus having a corona generating device with at least one corona wire positioned in closely spaced relation to -a Xerographic plate for applying an electrostatic charge onto the xerographic plate, and wherein the charging current to the xerographic plate is produced by the potential applied to the corona wire whereby an increase in potential on this wire will effect an increase in charging current and a decrease in this potential will effect a decrease in charging current;

a circuit for the corona generating device including a high voltage direct current source,

a first control tube having a cathode, an anode and a grid, .said cathode being electrically connected to said wire, said anode being electrically connected to the positive terminal of said source,

an electrical connection between the xerographic plate and the negative terminal of said source,

a resistor electrically connected in series between the xerographic plate and said negative terminal Where- -by the charging current through said xer-ographic plate flows through said resistor, said resistor being adapted to produce a voltage drop proportional to the charging current,

a voltage reference device electrically connected in series with said resistor for producing a differential voltage,

a second control tube having a cathode, an anode and a grid, said cathode, being electrically connected to the negativeterminal of said source, said anode being connected to the grid of said first tube,

means for conducting said differential voltage to the grid'ofv said second tube whereby tube differential voltage is amplified and conducted to the grid of said first tube,

said first tube being adapted to vary the voltage applied to said wire in accordance with said differential voltage.

References Cited UNITED STATES PATENTS 3,062,956 11/1962 Codichini 250-495 RALPH G. NILSON, Primary Examiner.

w. F. LINDQUIST, Assistant Examiner. 

1. IN A XEROGRAPHIC REPRODUCING APPARATUS HAVING A CORONA GENERATING DEVICE WITH AT LEAST ONE CORONA WIRE POSITIONED IN CLOSELY SPACED RELATION TO A XEROGRAPHIC PLATE FOR APPLYING AN ELECTROSTATIC CHARGE ONTO THE XEROGRAPHIC PLATE, AND WHEREIN THE CHARGING CURRENT TO THE XEROGRAPHIC PLATE IS PRODUCED BY THE POTENTIAL APPLIED TO THE CORONA WIRE WHEREBY AN INCREASE IN POTENTIAL ON THIS WIRE WILL EFFECT AN INCREASE IN CHARGING CURRENT AND A DECREASE IN THIS POTENTIAL WILL EFFECT A DECREASE IN CHARGING CURRENT; A CIRCUIT FOR THE CORONA GENERATING DEVICE INCLUDING A HIGH VOLTAGE DIRECT CURRENT SOURCE, A CONTROL TUBE HAVING A CATHODE, AN ANODE AND A GRID, SAID CATHODE BEING ELECTRICALLY CONNECTED TO SAID WIRE, SAID ANODE BEING ELECTRICALLY CONNECTED TO THE POSITIVE TERMINAL OF SAID SOURCE, AN ELECTRICAL CONNECTION BETWEEN THE XEROGRAPHIC AND THE NEGATIVE TERMINAL OF SAID SOURCE, 